Media curl is frequently considered one of the root causes of paper jams in paper handling and registration errors during rendering, and can be exacerbated by high-density images and plural color rendering issues. Media curl can be induced by several factors such as, for example, relative humidity, paper weight, paper size, sides imaged or the amount of data contained with a particular digital image.
Sheet curling can occur even in the context of unprinted sheets of paper due to changes in ambient humidity or the moisture content of the paper. Sheet curling can interfere with proper sheet feeding, causing sheet feeding jams, delays, misfeeds or registration errors. Sheet curling can cause media to have direct contact with printing cartridges and to damage the cartridges. If sheet curl is present in the output, it can interfere with proper stacking or other finishing operations. The amount of moisture in the sheet of paper can drastically change from the rendering process itself, to cause or exacerbate curl.
The sheet curl problem can also occur in duplex printing, when the sheets are re-fed or re-circulated for rendering imaging material on their second sides, especially if that involves a second pass of the sheet through a thermal fuser and/or higher density images on one side than the other. The media curl must be measured and controlled so that reliable marking can be achieved and damage to an ink cartridge can be prevented.
Various media curl sensors and control systems are known in the electro photographic rendering arts. Such prior art systems typically employ a single cross beam sensor for detecting the height/curl of the media. Such a sensor needs the nominal media transport speed as an input for the measurement of the paper curl, and an inaccurate nominal speed may lead to inaccurate curl readings. Also, such systems do not provide an effective mechanism and/or technique for measuring both the speed and the media curl height when the media passes the sensor.
Based on the foregoing, it is believed that a need exists for a dual cross beam sensor system and an improved method for measuring a lead edge and/or trail edge media curl. A need also exists for simultaneously measuring a true media speed and a curl height, as described in greater detail herein.